FreeChainXenon/gcc
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

libstdc++: Reduce std::variant template instantiation depth

Browse source 
The recursively defined constraints on _Variadic_union's user-defined
destructor (used for maintaining trivial destructibility of the variant
iff all of its alternatives are) turn out to require a template
instantiation depth of 3x the number of variants in C++20 mode, with the
instantiation stack looking like

  ...
  _Variadic_union<B, C, ...>
  std::is_trivially_destructible_v<_Variadic_union<B, C, ...>>
  _Variadic_union<A, B, C, ...>::~_Variadic_union()
  _Variadic_union<A, B, C, ...>
  ...

Ideally the template depth should be ~equal to the number of variants
(plus a constant).  Luckily it seems we don't need to compute trivial
destructibility of the alternatives at all from _Variadic_union, since
its only user _Variant_storage already has that information.  To that
end this patch removes these recursive constraints and instead passes
this information down from _Variant_storage.  After this patch, the
template instantiation depth for 87619.cc in C++20 mode is ~270 instead
of ~780.

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::_Variadic_union):
	Add bool __trivially_destructible template parameter.
	(__detail::__variant::_Variadic_union::~_Variadic_union):
	Use __trivially_destructible in constraints instead.
	(__detail::__variant::_Variant_storage): Pass
	__trivially_destructible value to _Variadic_union.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
This commit is contained in:
Patrick Palka 2024-01-15 16:41:42 -05:00
parent 731444b3c3
commit 2d55d94e5d
1 changed files with 7 additions and 8 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

15
libstdc++-v3/include/std/variant
View file

@ -392,7 +392,7 @@ namespace __variant
};
// Defines members and ctors.
template<typename... _Types>
template<bool __trivially_destructible, typename... _Types>
union _Variadic_union
{
_Variadic_union() = default;
@ -401,8 +401,8 @@ namespace __variant
_Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
};
template<typename _First, typename... _Rest>
union _Variadic_union<_First, _Rest...>
template<bool __trivially_destructible, typename _First, typename... _Rest>
union _Variadic_union<__trivially_destructible, _First, _Rest...>
{
constexpr _Variadic_union() : _M_rest() { }
@ -427,13 +427,12 @@ namespace __variant
~_Variadic_union() = default;
constexpr ~_Variadic_union()
requires (!is_trivially_destructible_v<_First>)
|| (!is_trivially_destructible_v<_Variadic_union<_Rest...>>)
requires (!__trivially_destructible)
{ }
#endif
_Uninitialized<_First> _M_first;
_Variadic_union<_Rest...> _M_rest;
_Variadic_union<__trivially_destructible, _Rest...> _M_rest;
};
// _Never_valueless_alt is true for variant alternatives that can
@ -514,7 +513,7 @@ namespace __variant
return this->_M_index != __index_type(variant_npos);
}
_Variadic_union<_Types...> _M_u;
_Variadic_union<false, _Types...> _M_u;
using __index_type = __select_index<_Types...>;
__index_type _M_index;
};
@ -552,7 +551,7 @@ namespace __variant
return this->_M_index != static_cast<__index_type>(variant_npos);
}
_Variadic_union<_Types...> _M_u;
_Variadic_union<true, _Types...> _M_u;
using __index_type = __select_index<_Types...>;
__index_type _M_index;
};